1. Field of the Invention
The invention discloses new drilling fluids based on ester oils and invert drilling muds built up thereon, which are distinguished by high ecological acceptability and at the same time good keeping and application properties. An important area of application for the new drilling fluid systems is in off-shore wells for the development of petroleum and/or natural gas deposits, the aim of the invention being particularly to make available technically usable drilling fluids with high ecological acceptability. The use of the new drilling fluid systems has particular significance in the marine environment, but is not limited thereto. The new mud systems can be put to quite general use even in land-based drilling, for example, in geothermal wells, water boreholes, in the drilling of geoscientific bores and in drilling for the mining industry. Here too it is essentially true that associated ecotoxic problems are substantially simplified by the ester-based drilling oil fluids selected according to the invention.
2. Discussion of Related Art
Oil-base drilling fluids are generally used as so-called invert-emulsion muds, which consist of a three-phase system; oil, water and finely particulate solids. These are preparations of the W/O emulsion type, i.e. the aqueous phase is distributed as a heterogeneous fine dispersion in the continuous oil phase. A number of additives can be used to stabilize the system as a whole and to adjust the described application properties, particularly emulsifiers or emulsifier systems, weighting agents, fluid-loss additives, alkali reserves, viscosity regulators and the like. For details, refer, e.g., to the publication by P. A. Boyd et al. "New Base Oil Used in Low-Toxicity Oil Muds" Journal of Petroleum Technology, 1985, 137 to 142, and R. B. Bennett, "New Drilling Fluid Technology--Mineral Oil Mud" Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited therein.
The relevant technology has for some time recognized the importance of ester-based oil phases in reducing the problems caused by such oil-base muds. For example, U.S. Pat. Nos. 4,374,737 and 4,481,121 disclose oil-base drilling fluids in which non-polluting oils are to be used. The following are mentioned as non-polluting oils of equal value --mineral oil fractions free from aromatic hydrocarbons and vegetable oils such as peanut oil, soybean oil, linseed oil, corn oil, rice oil or even oils of animal origin, such as whale oil. Without exception, these named ester oils of vegetable and animal origin are triglycerides of natural fatty acids, which are known to have a high environmental acceptability, and are clearly superior from the ecological viewpoint when compared with hydrocarbon fractions--even when these are free from aromatic hydrocarbons.
Interestingly enough, however, none of the examples in the aforementioned U.S. Patent Specifications describes the use of such natural ester oils in invert-drilling fluids such as those in question here. In all cases, mineral oil fractions are used as the continuous oil phase. Oils of vegetable and/or animal origin are not considered for reasons of practicality. The rheological properties of such oil phases cannot be controlled for the wide temperature range reaching from 0.degree. to 5.degree. C. on the one hand up to 250.degree. C. on the other hand, which is widely required in the industry.
In use, ester oils of the type in question do not in fact behave in the same way as the mineral oil fractions based on pure hydrocarbon which were previously used. Ester oils are subject to partial hydrolysis in practical use also and particularly in W/O invert drilling muds. Free carboxylic acids are formed as a result. The earlier Applications P 38 42 659.5 and P 38 42 703.6 (U.S. Ser. No. 07/452,457 titled "The Use of Selected Ester Oils in Drilling Fluids and Muds" and U.S. Ser. No. 07/452,988 titled "Drilling Fluids and Muds Containing Selected Ester Oils") described the problems caused thereby and give suggestions for their solution.
The subject of these earlier Applications is the use of ester oils based on selected monocarboxylic acids or monocarboxylic acid mixtures and monofunctional alcohols with at least 2, and preferably with at least 4 carbon atoms. The earlier applications show that with the esters and ester mixtures they disclose, monofunctional reactants can not only be given satisfactory rheological properties in fresh drilling fluid, but that by using selected known alkali reserves it is possible to work with the drilling fluid and in this way to inhibit undesirable corrosion. For the alkali reserve, one can add calcium hydroxide or lime and/or use zinc oxide or comparable zinc compounds. However, in this case an additional restriction is advisable. If undesired thickening of the oil-base invert-mud system is to be prevented in practical use, the amount of alkalizing additive and in particular the amount of lime must be limited. The maximum amount provided is set at about 2 lb/bbl oil-base mud in the disclosure of the aforementioned earlier Applications.